Adaptive error correction in a memory system

ABSTRACT

According to one aspect, a method for adaptive error correction in a memory system includes reading data from a memory array of a non-volatile memory device in the memory system. Error correcting logic checks the data for at least one error condition stored in the memory array. Based on determining that the at least one error condition exists, a write-back indicator is asserted by the error correcting logic to request correction of the at least one error condition, where the write-back indicator is a discrete signal sent to a memory controller, and the at least one non-volatile memory device asserting the write-back indicator extends cycle timing monitored by the memory controller while the write-back indicator is asserted. Based on determining that the at least one error condition does not exist, accesses of the memory array continue without asserting the write-back indicator.

DOMESTIC PRIORITY

This application is a continuation of U.S. patent application Ser. No.14/446,922, filed Jul. 30, 2014, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates generally to computer memory, and moreparticularly, to adaptive error correction in a memory system.

Computer systems often require a considerable amount of high speedmemory to hold information, such as data and programs, when a computeris powered and operational. This information is normally binary,composed of patterns of 1's and 0's known as bits of data. The bits ofdata are typically stored in separate cells of memory devices. Currentand emerging memory and storage technologies may experience single cellbit errors during a storage interval. In dynamic random access memory(DRAM) technology, these types of errors can be caused by cell leakage,alpha particle collision events, variable retention phenomena andseveral other means. DRAM is a type of volatile memory that usescapacitors to store bits as levels of charge. DRAM requires periodicrefreshing to maintain correct charge levels within memory cells. DRAMread operations are typically destructive in that the process of readingcell values can change the charge level per cell. To restore the correctcharge level as part of a read operation, a write-back operation isperformed such that the intended bit state of each cell is maintained.

Current state of the art systems typically resolve memory bit errorsthrough error correction techniques at the system level. As technologyscaling results in denser memory cell counts per memory device, singlecell error rates are likely to increase in memory systems.

SUMMARY

According to one embodiment, a method for adaptive error correction in amemory system includes reading data from a memory array of anon-volatile memory device in the memory system. Error correcting logicchecks the data for at least one error condition stored in the memoryarray. Based on determining that the at least one error conditionexists, a write-back indicator is asserted by the error correcting logicto request correction of the at least one error condition, where thewrite-back indicator is a discrete signal sent to a memory controller,and the at least one non-volatile memory device asserting the write-backindicator extends cycle timing monitored by the memory controller whilethe write-back indicator is asserted. Based on determining that the atleast one error condition does not exist, accesses of the memory arraycontinue without asserting the write-back indicator.

According to another embodiment, a memory system includes a memorycontroller and at least one non-volatile memory device that includes amemory array and error correcting logic. The at least one non-volatilememory device is operably coupled to the memory controller andconfigured to read data from the memory array. The at least onenon-volatile memory device is further configured to check the data forat least one error condition stored in the memory array and assert awrite-back indicator to request correction of the at least one errorcondition based on determining that the at least one error conditionexists, where the write-back indicator is a discrete signal sent to amemory controller, and the at least one non-volatile memory deviceasserting the write-back indicator extends cycle timing monitored by thememory controller while the write-back indicator is asserted. Accessesof the memory array continue without asserting the write-back indicatorbased on determining that the at least one error condition does notexist.

According to a further embodiment, a computer program product foradaptive error correction in a memory system is provided. The computerprogram product includes a computer readable storage medium havingprogram instructions embodied therewith, the program instructionsreadable by a processing circuit to cause the processing circuit toperform a method. The method includes reading data from a memory arrayof a non-volatile memory device in the memory system and checking thedata for at least one error condition stored in the memory array. Basedon determining that the at least one error condition exists, awrite-back indicator is asserted to request correction of the at leastone error condition, where the write-back indicator is a discrete signalsent to a memory controller, and the at least one non-volatile memorydevice asserting the write-back indicator extends cycle timing monitoredby the memory controller while the write-back indicator is asserted.Based on determining that the at least one error condition does notexist, accesses of the memory array continue without asserting thewrite-back indicator.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a block diagram of a system operable to performadaptive error correction in accordance with an embodiment;

FIG. 2 illustrates a block diagram of a memory system operable toperform adaptive error correction in accordance with an embodiment;

FIG. 3 illustrates an example of cycle timing adjustment based on awrite-back request in accordance with an embodiment;

FIG. 4 illustrates another example of a block diagram of a memory systemoperable to perform adaptive error correction in accordance with anembodiment;

FIG. 5 illustrates an example of write back command timing relative to aread cycle in accordance with an embodiment;

FIG. 6 illustrates an example of multiple write-back buffer entries andan associated threshold in accordance with an embodiment;

FIG. 7 illustrates an example of a burst read data format in accordancewith an embodiment;

FIG. 8 illustrates another example of a block diagram of a memory systemoperable to perform adaptive error correction in accordance with anembodiment;

FIG. 9 illustrates an example of a state transition diagram for a writeoperation in accordance with an embodiment; and

FIG. 10 illustrates a flow diagram of a method for performing adaptiveerror correction in a memory system in accordance with an embodiment.

DETAILED DESCRIPTION

An embodiment is directed to performing adaptive error correction in amemory system that includes non-volatile memory devices. Volatile memorysuch as DRAM may require a write-back operation after each readoperation to ensure that memory cells in the DRAM are restored after adestructive read operation. Many non-volatile memory devices performnon-destructive read operations, in that reading the non-volatile memorydevices does not alter the state of the memory cells. Unlike volatilememory devices, non-volatile memory can retain memory cell state valuesthrough cycling of power off-and-on. Examples of non-volatile memorydevices that can incorporate embodiments as described herein includephase-change memory (PCM), magnetoresistive random-access memory (MRAM),ferroelectric RAM (FeRAM), nonvolatile static RAM (nvSRAM), andrestrictive RAM (ReRAM), as well as other non-volatile memorytechnologies known in the art. Non-volatile memory can degrade overtime, under thermal loads, or due to other effects such that one or morememory cell values can change from a ‘0’ to a ‘1’ or vice versa.

In exemplary embodiments, one or more non-volatile memory devices eachinclude localized error correcting logic to perform localized errorchecking prior to returning data to a memory controller. The errorcorrecting logic can use an error-correcting code to locally correct theerrant data within the non-volatile memory device. Upon checking data inthe non-volatile memory device for an error, a write-back indicator canbe asserted to request error correction. The write-back indicator can besent back to a memory controller to either indicate that a write-backcommand must be issued or as a status of a memory device initiated writeback. Where the memory device initiates the write-back locally, acurrent cycle time may be extended. For instance, during a read cycle,the read cycle timing can be extended by the memory device initiatingthe write-back. Alternatively, the memory device can internally retainthe write-back indicator and upon receiving a write-back command fromthe memory controller, the memory device can perform a write-back if itsrespective write-back indicator is set.

When no error conditions are identified, the write-back indicator is notset and normal access to the memory device can continue. Thus, for caseswhere a read cycle is extended based on the write-back indicator beingset, the extended cycle time for a write-back operation only occurs whena write-back is needed rather than performing a write-back after everyread operation.

FIG. 1 illustrates a block diagram of a system 100 operable to performadaptive error correction in accordance with an embodiment. The system100 depicted in FIG. 1 includes a computer processor 102, memory 106including multiple non-volatile memory devices, and a memory controller104 for receiving data from the computer processor 102 to be stored inthe memory 106. Collectively, the memory controller 104 and the memory106 are referred to as a memory system 105. In an embodiment, thenon-volatile memory devices of the memory 106 are arranged into ranks orgroups of non-volatile memory devices accessible at the same time.

In one embodiment the memory controller 104 is coupled to the computerprocessor 102 and receives write requests from the computer processor102. The write requests contain data to be written to the memory 106 anda logical address for identifying the location in the memory 106 towhich the data will be written. The memory controller 104 stores data ata physical address within the memory 106. In an embodiment, the memorycontroller 104 maps the logic address to a physical address in thememory 106 when storing or retrieving data.

The system 100 is one example of a configuration that may be utilized toperform the processing described herein. Although the system 100 hasbeen depicted with only a memory 106, memory controller 104, andcomputer processor 102, it will be understood that other embodimentswould also operate in other systems including additional elements, e.g.,multiple computers processors 102 and multiple levels of memory 106. Inan embodiment, the memory 106, memory controller 104, and computerprocessor 102 are not located within the same computer. For example, thememory 106 and memory controller 104 may be located in one physicallocation (e.g., on a memory module) while the computer processor 102 islocated in another physical location (e.g., the computer processor 102accesses the memory controller 104 via a network). In addition, portionsof the processing described herein may span one or more of the memory106, memory controller 104, and computer processor 102.

FIG. 2 illustrates a block diagram of a memory system 200 operable toperform adaptive error correction in accordance with an embodiment. Thememory system 200 is an embodiment of the memory system 105 of FIG. 1.In the example of FIG. 2, memory controller 202 is operably coupled tonon-volatile memory devices 204A-204N. Command, address, and data buses206 enable read and write commands to be issued from the memorycontroller 202 to the non-volatile memory devices 204A-204N. Awrite-back request 208 may be driven as a discrete signal output fromthe non-volatile memory devices 204A-204N as a write-back indicator tothe memory controller 202. The write-back request 208 may be an opendrain pull up dotted OR (indicated as pull up resistor 210) acrossseveral of the non-volatile memory devices 204A-204N. One or more of thememory devices 204A-204N can drive the write-back request 208 byasserting a write-back indicator based on detecting at least one errorcondition stored in a memory array 212. Each of the non-volatile memorydevices 204A-204N can include a memory array 212 that stores data 214and check data 216. The check data 216 may be error-correcting code(ECC) check data that can be used by error correcting logic 218 in eachof the non-volatile memory devices 204A-204N to detect and correcterrors. For example, the error correcting logic 218 can compute anerror-correcting code of the data 214 read from the memory array 212 andread check data 216 from the memory array 212. A mismatch between theerror-correcting code computed upon reading the data 214 and the checkdata 216 read from the memory array 212 can identify an error. Thisresults in asserting a write-back indicator that drives the write-backrequest 208.

The check data 216 can be used to compute a corrected version of thedata 214 to report back to the memory controller 202 and to be writtenback to the memory array 212. The data 214 and check data 216 caninclude any number of individual cells or bits. A write-back operationcan be performed on an individual memory device basis such that any ofthe non-volatile memory devices 204A-204N identifying an error cancorrect respective data 214. Error detection and correction onwrite-back can be performed on a word basis, a page basis, a bank basis,or any subdivision unit basis within each of the non-volatile memorydevices 204A-204N. When a memory device, such as non-volatile memorydevice 204A, determines that a write-back is needed while performing aread cycle, the non-volatile memory device 204A can assert thewrite-back request 208 while performing the write-back of the correcteddata. This may result in extending the current cycle time at the memorycontroller 202.

FIG. 3 illustrates an example of cycle timing adjustment based on awrite-back request in accordance with an embodiment. FIG. 3 is describedwith further reference to FIG. 2. The memory controller 202 initiates aread cycle by driving a read cycle signal of the buses 206 to a lowstate 302 (i.e., asserted in this example). Under normal operatingconditions, when no error is detected by the error correcting logic 218,then the write-back indicator is not asserted on the write-back request208, the data 214 is returned to the memory controller 202, and normaloperation continues where further accesses of the memory array 212 canbe made on subsequent commands. A read cycle normally ends at apredictable time, and the memory controller 202 sets the read cycle to ahigh state 304 (i.e., not asserted in this example) after apredetermined cycle time interval. If the error correcting logic 218detects at least one error condition upon reading the memory array 212in response to a read command, the error correcting logic 218 can asserta write-back indicator that drives the write-back request 208 from ahigh state 306 (i.e., not asserted in this example) to a low state 308(i.e., asserted in this example). Assertion of the write-back request208 can result in extending cycle timing of the read cycle which timeshifts a transition 310 between the low state 302 and high state 304 ofthe read cycle. Since the cycle time extension only occurs on detectederror conditions resulting in a write-back, the additional read cyclecompletion latency is only occasionally incurred rather than performinga write-back as part of every read operation.

FIG. 4 illustrates another example of a block diagram of a memory system400 operable to perform adaptive error correction in accordance with anembodiment. Similar to the memory system 200 of FIG. 2, the memorysystem 400 of FIG. 4 is an embodiment of the memory system 105 of FIG. 1and includes a memory controller 402 operably coupled to non-volatilememory devices 404A-404N. Command, address, and data buses 406 enableread and write commands to be issued from the memory controller 402 tothe non-volatile memory devices 404A-404N. Rather than providing aseparate write-back request, such as the write-back request 208 of FIG.2, the memory system 400 does not include a dedicated discrete output tobe driven in response to a write-back indicator 408 generated by one ormore of the non-volatile memory devices 404A-404N.

Each of the non-volatile memory devices 404A-404N can include a memoryarray 412 that stores data 414 and check data 416. The check data 416may be error-correcting code (ECC) check data that can be used by errorcorrecting logic 418 in each of the non-volatile memory devices404A-404N to detect and correct errors. For example, the errorcorrecting logic 418 can compute an error-correcting code of the data414 read from the memory array 412 and read check data 416 from thememory array 412. A mismatch between the error-correcting codecalculated upon reading the data 414 and the check data 416 read fromthe memory array 412 can identify an error. This results in assertingthe write-back indicator 408. A write-back buffer 410 may be included ineach of the non-volatile memory devices 404A-404N to store a correctedversion of the data 414 as write-back data. The write-back data can bewritten from the write-back buffer 410 to the memory array 412 based onreceiving a write-back command from the memory controller 402. In anembodiment where the write-back indicator 408 is not reported to thememory controller 402, the memory controller 402 can periodically issuea write back command to the non-volatile memory devices 404A-404N. In analternate embodiment, the write-back indicator 408 is sent to the memorycontroller 402 as an encoded message, such as a bit in a read databurst. Since the non-volatile memory devices 404A-404N buffer thewrite-back data, the memory controller 402 need not respond immediatelyto the write-back indicator 408 but can schedule a write-back commandfor a future time.

FIG. 5 illustrates an example of write-back command timing relative to aread cycle in accordance with an embodiment. FIG. 5 is described inreference to FIG. 4. The memory controller 402 initiates a read cycle bydriving a read cycle signal of the buses 406 to a low state 502 (i.e.,asserted in this example). Regardless of whether or not the write-backindicator 408 is asserted the timing of the read cycle remainsconsistent, and the memory controller 402 sets the read cycle to a highstate 504 (i.e., not asserted in this example) after a predeterminedcycle time interval. In the example of FIG. 5 as the read cycletransitions between the low state 502 and the high state 504, awrite-back command of the buses 406 remains at a high state 506 (i.e.,not asserted in this example). At some future time when the read cycleis at the high state 504, the memory controller 402 sends the write-backcommand to the non-volatile memory devices 404A-404N by driving thewrite-back command to a low state 508 (i.e., asserted in this example).This triggers any of the non-volatile memory devices 404A-404N having anasserted write-back indicator 408 to update data 414 in each respectivememory array 412 with write-back data from the respective write-backbuffer 410.

FIG. 6 illustrates an example of multiple write-back buffer entries 602and an associated threshold 604 in accordance with an embodiment. Withcontinued reference to FIG. 4, the write-back buffer 410 can includemultiple write-back buffer entries 602 associated with addresses thatresulted in an errant read. Rather than immediately notifying the memorycontroller 402 when at least one write-back is needed, the threshold 604can be used to define a threshold number of the write-back bufferentries 602 to populate before notifying the memory controller 402 thata write-back is needed. In the example of FIG. 6, the threshold 604 isset to trigger notification upon populating write-back buffer entrynumber ‘M’, where ‘M’ is a number between a first entry number (i.e.,entry 0) and a maximum entry number (i.e., entry N). Setting thethreshold 604 to an intermediate value allows more time to elapsebetween write-back operations while leaving one or more additionalbuffer entries available should the memory controller 402 delayscheduling of the write-back command. The threshold 604 can beconfigurable as part of a mode register to any value between ‘0’ and ‘N’in the example of FIG. 6.

In one embodiment, when a read command is issued to the same address asone of the write-back buffer entries 602, then the read data is returnedfrom the corresponding write-back buffer entry 602. Similarly, when awrite command is issued to the same address as one of the write-backbuffer entries 602, then the write data can be written directly writtento the memory and the corresponding write back buffer entry 602 can becleared.

FIG. 7 illustrates an example of a burst read data format 700 inaccordance with an embodiment. The burst read data format 700 is anexample where read data 702, check data 704, and a write-back indicator706 can be sent in response to a burst read command. With respect to theexample of FIG. 4, the burst read data format 700 allows the write-backindicator 408 to be encoded and sent on buses 406 as the write-backindicator 706. When there are no errors in the data 414 of FIG. 4, theread data 702 in the burst read data format 700 is a copy of a number ofbits from the data 414. When there is an error, the read data 702 can bea corrected version of the data 414. The check data 704 can be copiedfrom the check data 416 of FIG. 4 or generated for the particular sizingof the burst read data format 700. For instance, the check data 704 canbe cyclic redundancy check (CRC) bits, data bus inversion (DBI) bits,ECC bits, or another error checking format known in the art.

FIG. 8 illustrates another example of a block diagram of a memory system800 operable to perform adaptive error correction in accordance with anembodiment. Similar to the memory system 400 of FIG. 4, the memorysystem 800 of FIG. 8 is an embodiment of the memory system 105 of FIG. 1and includes a memory controller 802 operably coupled to non-volatilememory devices 804A-804N. Command, address, and data buses 806 enableread and write commands to be issued from the memory controller 802 tothe non-volatile memory devices 804A-804N.

Each of the non-volatile memory devices 804A-804N can include a memoryarray 812 that stores data 814 and check data 816. The check data 816may be error-correcting code (ECC) check data that can be used by errorcorrecting logic 818 in each of the non-volatile memory devices804A-804N to detect and correct errors. For example, the errorcorrecting logic 818 can compute an error-correcting code of the data814 read from the memory array 812 and read check data 816 from thememory array 812 as part of a write verify operation. A mismatch betweenthe error-correcting code calculated upon reading the data 814 and thecheck data 816 read from the memory array 812 can identify an error.This results in asserting the write-back indicator 808. A write retrybuffer 810 may be included in each of the non-volatile memory devices804A-804N to store write data to be written to the memory array 812based on receiving a write command from the memory controller 802. Thewrite data can be written from the write retry buffer 810 to the memoryarray 812 when executing a write command from the memory controller 802.Reading of the data 814 from the memory array 812 can be performed as awrite verify operation, where correction of at least one error conditionis performed as a write retry operation.

Rather than using the check data 816 for write verification, the writeverification may be performed by attempting to store write data from thewrite retry buffer 810, reading the data 814 after the write operation,and comparing the data 814 read from the memory array 812 to the writedata in the write retry buffer 810 to confirm that no bit differencesare detected. In one embodiment, the memory system 400 of FIG. 4 and thememory system 800 are the same, where the write-back buffer 410 of FIG.4 also functions as the write retry buffer 810 of FIG. 8.

In an embodiment where the write-back indicator 808 is not reported tothe memory controller 802, the memory controller 802 can periodicallyissue a write retry command to the non-volatile memory devices804A-804N. In an alternate embodiment, the write-back indicator 808 issent to the memory controller 802 as an encoded message, such as a bitin a data burst. Since the non-volatile memory devices 804A-804N bufferthe write data, the memory controller 802 need not respond immediatelyto the write-back indicator 808 but can schedule a write retry commandfor a future time.

FIG. 9 illustrates an example of a state transition diagram 900 for awrite operation in accordance with an embodiment. FIG. 9 is describedwith continued reference to FIG. 8. When a write command is received thenon-volatile memory devices 804A-804N at write state 902, thenon-volatile memory devices 804A-804N transition to a write verify state904 to read the memory array 812 and confirm that the write wassuccessful. If the write failed, the non-volatile memory devices804A-804N transition to a write retry state 906 to attempt the writeagain using write data in the write retry buffer 810. After attempting awrite retry, the non-volatile memory devices 804A-804N return to thewrite verify state 904. If at the write verify state 904 the write isdeemed successful, the non-volatile memory devices 804A-804N transitionto a write complete state 908 and further read/write operations can beperformed. The state transitions in the state transition diagram 900 canbe driven by the memory controller 802 or internally managed within thenon-volatile memory devices 804A-804N.

FIG. 10 illustrates a flow diagram of a method 1000 for performingadaptive error correction in accordance with an embodiment. The depictedblocks may be part of or in addition to another process and/or may beperformed in any suitable order to provide adaptive error correction.The method 1000 can be implemented for a variety of memory systemconfigurations and is described in reference to FIGS. 1-10. Forinstance, the method 1000 can be performed by non-volatile memorydevices 204A-N, 404A-N, and/or 804A-N.

At block 1002, data are read from a memory array of a non-volatilememory device in a memory system, such as data 214 from memory array 212of non-volatile memory device 202A in memory system 200. At block 1004,error correcting logic, such as error correcting logic 218, can checkthe data for at least one error condition stored in the memory array.For example, an error-correcting code of the data 214 can be computedand error-correcting code check data 216 may be read from the memoryarray 212. The error correcting logic 218 can identify that the at leastone error condition exists based on a mismatch between theerror-correcting code of the data 214 and the error-correcting codecheck data 216 from the memory array 212. The error-correcting codecheck data 216 read from the memory array 212 can be used to generate acorrected version of the data 214 to write-back to the memory array 212at a desired time.

At block 1006, it is determined whether at least one error conditionexists in the data read from the memory array. At block 1008, based ondetermining that the at least one error condition exists, a write-backindicator is asserted by the error correcting logic to requestcorrection of the at least one error condition. For example, the errorcorrecting logic 218 can cause the write-back request 208 to be drivento a low state 308 (i.e., asserted) to notify the memory controller 202of the write-back. At block 1010, based on determining that the at leastone error condition does not exist, accesses of the memory arraycontinue without asserting the write-back indicator. Thus, read cycletiming of transition 310 need not be modified where no write-back isneeded.

As in the example of FIG. 2, a write-back indicator can be sent from oneor more of the non-volatile memory devices 204A-204N to the memorycontroller 202 of the memory system 200 as a discrete signal output,e.g., write-back request 208. Alternatively, as in the example of FIG.7, the write-back indicator 706 can be encoded as a bit that is embeddedin burst read data returned to the memory controller from thenon-volatile memory device in response to a read command. A non-volatilememory device can extend cycle timing monitored by the memory controllerwhile the write-back indicator is asserted, such as a read cycle orwrite verify cycle.

In an embodiment that includes a write-back buffer in a non-volatilememory device, such as write-back buffer 410 of FIG. 4, a correctedversion of the data can be stored as write-back data in the write-backbuffer. The corrected version of the data can change the state of one ormore flipped bits using known error correction algorithms in combinationwith the error-correcting code check data from the memory array. Basedon receiving a write-back command from the memory controller, thewrite-back data can be written to the memory array. Alternatively, thenon-volatile memory device can determine when to perform the write-back.The write-back buffer can include multiple write-back buffer entries.The non-volatile memory device may notify the memory controller that awrite-back is needed based on determining that a threshold number ofwrite buffer entries is populated, such as threshold 604 relative to thewrite-back buffer entries 602 of FIG. 6. The non-volatile memory devicecan then wait for the memory controller to respond with the write-backcommand prior to writing the write-back data to the memory array.

In one embodiment, a non-volatile memory device can include a writeretry buffer, such as the write retry buffer 810 of FIG. 8, to supportrewrite operations on a write failure. Write data can be saved into awrite retry buffer based on receiving a write command for thenon-volatile memory device. A write of the write data can be executed tostore the write data in the memory array. The reading of the data fromthe memory array can be performed as a write verify operation, wherecorrection of the at least one error condition is performed as a writeretry operation. A write retry can be initiated as needed orperiodically by the memory controller.

Technical effects include performing adaptive error correction in amemory system by detecting when a write-back is needed on an errorcondition adaptively responding by extending a read or write verifycycle to correct the error, providing notification of the need for thewrite-back to a memory controller, and/or responding to a write-backcommand if a write-back indicator is set.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention. The computer readable storage medium can be atangible device that can retain and store instructions for use by aninstruction execution device. The computer readable storage medium maybe, for example, but is not limited to, an electronic storage device, amagnetic storage device, an optical storage device, an electromagneticstorage device, a semiconductor storage device, or any suitablecombination of the foregoing. A non-exhaustive list of more specificexamples of the computer readable storage medium includes the following:a portable computer diskette, a hard disk, a random access memory (RAM),a read-only memory (ROM), an erasable programmable read-only memory(EPROM or Flash memory), a static random access memory (SRAM), aportable compact disc read-only memory (CD-ROM), a digital versatiledisk (DVD), a memory stick, a floppy disk, a mechanically encoded devicesuch as punch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method for adaptive error correction in amemory system, the method comprising: reading data from a memory arrayof a non-volatile memory device in the memory system; checking, by errorcorrecting logic, the data for at least one error condition stored inthe memory array; based on determining that the at least one errorcondition exists, asserting a write-back indicator, by the errorcorrecting logic, to request correction of the at least one errorcondition, wherein the write-back indicator is a discrete signal sent toa memory controller, and the at least one non-volatile memory deviceasserting the write-back indicator extends cycle timing monitored by thememory controller while the write-back indicator is asserted; and basedon determining that the at least one error condition does not exist,continuing accesses of the memory array without asserting the write-backindicator.
 2. The method of claim 1, wherein determining that the atleast one error condition exists comprises: computing anerror-correcting code of the data; reading error-correcting code checkdata from the memory array; and identifying that the at least one errorcondition exists based on a mismatch between the error-correcting codeof the data and the error-correcting code check data from the memoryarray.
 3. The method of claim 1, further comprising: storing a correctedversion of the data as write-back data in a write-back buffer; andwriting the write-back data to the memory array based on receiving awrite-back command from a memory controller.
 4. The method of claim 3,wherein the write-back buffer comprises multiple write-back bufferentries, and the method further comprising: notifying the memorycontroller that a write-back is needed based on determining that athreshold number of write buffer entries is populated; and waiting forthe memory controller to respond with the write-back command prior towriting the write-back data to the memory array.
 5. The method of claim1, further comprising: saving write data into a write retry buffer basedon receiving a write command for the non-volatile memory device;executing a write of the write data to store the write data in thememory array; and performing the reading of the data from the memoryarray as a write verify operation, wherein correction of the at leastone error condition is performed as a write retry operation.
 6. A memorysystem, comprising: a memory controller; and at least one non-volatilememory device comprising a memory array and error correcting logic, theat least one non-volatile memory device operably coupled to the memorycontroller and configured to: read data from the memory array; check thedata for at least one error condition stored in the memory array; asserta write-back indicator to request correction of the at least one errorcondition based on determining that the at least one error conditionexists, wherein the write-back indicator is a discrete signal sent to amemory controller, and the at least one non-volatile memory deviceasserting the write-back indicator extends cycle timing monitored by thememory controller while the write-back indicator is asserted; andcontinue accesses of the memory array without asserting the write-backindicator based on determining that the at least one error conditiondoes not exist.
 7. The memory system of claim 6, wherein the check ofthe data for the at least one error condition stored in the memory arraycomprises: computing an error-correcting code of the data; readingerror-correcting code check data from the memory array; and identifyingthat the at least one error condition exists based on a mismatch betweenthe error-correcting code of the data and the error-correcting codecheck data from the memory array.
 8. The memory system of claim 6,wherein the at least one non-volatile memory device is furtherconfigured to: store a corrected version of the data as write-back datain a write-back buffer; and write the write-back data to the memoryarray based on receiving a write-back command from the memorycontroller.
 9. The memory system of claim 8, wherein the write-backbuffer comprises multiple write-back buffer entries, and the at leastone non-volatile memory device is further configured to: notify thememory controller that a write-back is needed based on determining thata threshold number of write buffer entries is populated; and wait forthe memory controller to respond with the write-back command prior towriting the write-back data to the memory array.
 10. The memory systemof claim 6, wherein the at least one non-volatile memory device isfurther configured to: save write data into a write retry buffer basedon receiving a write command; execute a write of the write data to storethe write data in the memory array; and perform the read of the datafrom the memory array as a write verify operation, wherein correction ofthe at least one error condition is performed as a write retryoperation.
 11. A computer program product for adaptive error correctionin a memory system, the computer program product comprising: a computerreadable storage medium having program instructions embodied therewith,the program instructions readable by a processing circuit to cause theprocessing circuit to perform a method comprising: reading data from amemory array of a non-volatile memory device in the memory system;checking the data for at least one error condition stored in the memoryarray; based on determining that the at least one error conditionexists, asserting a write-back indicator to request correction of the atleast one error condition, wherein the write-back indicator is adiscrete signal output by the non-volatile memory device to a memorycontroller of the memory system, and the non-volatile memory deviceextends cycle timing monitored by the memory controller while thewrite-back indicator is asserted; and based on determining that the atleast one error condition does not exist, continuing accesses of thememory array without asserting the write-back indicator.
 12. Thecomputer program product of claim 11, wherein determining that the atleast one error condition exists comprises: computing anerror-correcting code of the data; reading error-correcting code checkdata from the memory array; and identifying that the at least one errorcondition exists based on a mismatch between the error-correcting codeof the data and the error-correcting code check data from the memoryarray.
 13. The computer program product of claim 11, wherein the programinstructions are further configured to cause the processing circuit toperform the method comprising: storing a corrected version of the dataas write-back data in a write-back buffer; and writing the write-backdata to the memory array based on receiving a write-back command from amemory controller.
 14. The computer program product of claim 13, whereinthe write-back buffer comprises multiple write-back buffer entries, andthe program instructions are further configured to cause the processingcircuit to perform the method comprising: notifying the memorycontroller that a write-back is needed based on determining that athreshold number of write buffer entries is populated; and waiting forthe memory controller to respond with the write-back command prior towriting the write-back data to the memory array.
 15. The computerprogram product of claim 11, wherein the program instructions arefurther configured to cause the processing circuit to perform the methodcomprising: saving write data into a write retry buffer based onreceiving a write command for the non-volatile memory device; executinga write of the write data to store the write data in the memory array;and performing the reading of the data from the memory array as a writeverify operation, wherein correction of the at least one error conditionis performed as a write retry operation.